The invention relates to a method for the quantitative determination of deuterium in samples containing hydrogen, and to a reduction oven for reducing gaseous or liquid substances containing hydrogen, which may be coupled directly to an isotope ratio mass spectrometer (IRMS), thereby permitting an online-coupled, reliable deuterium determination which can be automatized.
In deuterium determination using an IRMS, various sample-chemical methods for generating hydrogen are known. In each case, the mass-spectrometric isotope analysis is performed using hydrogen gas which must be generated from each sample either by reduction or isotope exchange between water and hydrogen. Most widely known are offline methods involving separation of sample chemistry and measurement. The hydrogen gas is liberated by means of reducing agents such as iron, manganese, zinc, uranium and chromium, where no satisfactory results are achieved when using iron and manganese.
According to the well-known zinc method (Coleman M. L., Sherpherd T. J., Durham J. J., Rouse J. E., Moore G. R., Anal. Chem. 1982, 54, 993-995, and Kendall C. and Coplen T. B., Anal. Chem. 1985, 57, 1437-1440; and Tanweer A., Anal. Chem. 1990, 62, 2158-2160 and Vennemann T. W. and O'Neil J. R., Chem. Geol. (Isot. Geosci. Sect.) 1993, 103, 227-234), the reduction of water using zinc is effected at about 400.degree. C. in ampoules or in a circulation system. Depending on the method, the sample quantity required is 5-70 mg of pure water.
Disadvantageously, only zinc made using a special production process provides satisfactory results. If the water contains contaminations (oil, salt etc.), this method cannot be applied. In addition, the isotope ratio is distorted by the solubility of hydrogen in zinc.
Furthermore, the use of uranium as reducing agent is well-known. Water is reduced by uranium at various temperatures (400-800.degree. C.) in a special apparatus. The sample quantity is analogous to that in the above-mentioned zinc method (Bigeleisen J., Perlmann M. L., H. C. Anal. Chem. 1952, 24, 1356-1357, and GB patent 904,165, and GSF Jahresbericht 1987, 217-225).
Disadvantageously, specific temperature conditions must be maintained precisely because the solubility of hydrogen in uranium is temperature-dependent. Deviations in temperature result in irreversible formation of uranium hydride (distorting the isotope ratio). The use of contaminated water (oil, salt etc.) is not possible.
Reduction using chromium has also been suggested (Rolle W., Hubner H., Fresenius Z., Anal. Chemie 232 [1967] 328, and Runge A., Isotopenpraxis 16 [1980] 2). The reduction of water and other hydrogen-containing substances by chromium is effected at temperatures between 700 and 1000.degree. C. in ampoules or special apparatus. The sample quantity required is 5 mg of water or an amount of hydrogen from another hydrogen-containing substance, which is equivalent to said quantity of water.
Likewise, online methods using coupling to the IRMS are known. According to Horita, J. Chem. Geol. (Isot. Geosci. Sect.) 1988, 72, 89-94, and in J. Chem. Geol. (Isot. Geosci. Sect.) 1989, 79, 107-112, and Horita J., Ueda A., Mizukami K. and Takatori I., Appl. Radiat. Isot. 40 (1989) 9, 801-805), an isotope exchange method is known. In this method, the isotope equilibrium between a water sample and hydrogen gas is established using a platinum catalyst, and the sample is directly introduced into the IRMS.
This method is disadvantageous in that large quantities of sample (20 ml of pure water) are required, constant temperature (.ltoreq.0.03.degree. C.) during exchange must be ensured because of the temperature dependence of the isotope separation factor, and a great deal of time is required in sample preparation, so that this method is difficult to handle and has found only minor use.
According to ZFI-Mitteilungen 37 (1981), pp. 19-52, and GB patent 904,165, and GSF-Jahresberichte 1985-1987, it is also well-known to use the reduction process for online procedures. The coupled methods of sample preparation suggested so far, involving mass spectrometer and TRMS, respectively, use uranium as reducing agent. However, these methods permit only reduction of water. Other hydrogen-containing substances cannot be reduced. In addition, the set-up of equipment is not uncomplicated, larger quantities of sample are required, and commercially available autosamplers cannot be used because of these sample quantities.
Thus, the method described in ZFI-Mitteilungen 37 (1981), pp. 49-52, involves complete evaporation of a water sample in a pre-connected vessel and subsequent reduction of a part of the water vapor on uranium in a post-connected apparatus, before the gas to be measured is introduced into the mass spectrometer to determine the isotope ratio (D/H) (cf., FIG. 16).
In addition to the problems of using uranium as reducing agent, which were already mentioned, the measurings procedure of determining the isotope ratio by integrating the area of the measured signals in this latter case can also be referred to as disadvantageous.